Particulate matter may form in vehicle exhaust when less than complete combustion of an air-fuel mixture occurs within an internal combustion engine. In some examples, the particulate matter may be trapped in a filter for oxidation at a later time so that the particulate matter may be converted to CO2. However, the particulate filter trapping efficiency may degrade over time. One way to determine whether or not the efficiency of a particulate filter has degraded is to compare the amount of particulate matter entering the particulate filter against the amount of particulate matter exiting the particulate filter. Particulate matter sensors can provide an indication of the amount of particulate matter entering and exiting the particulate filter when particulate sensors are positioned upstream and downstream of the particulate filter in a direction of exhaust flow. However, it may be desirable to verify operation of the particulate matter sensors to ensure that the particulate filter efficiency as determined from the particulate matter sensors is reliable. Verifying operation of the downstream particulate sensor may be particularly difficult because the particulate filter tends to hold exhaust constitutents.
The inventors herein have recognized the above-mentioned disadvantages and have developed a method for verifying operation of particulate matter sensors. In one example, a method for diagnosing a particulate matter sensor comprises: injecting a liquid into an exhaust system before a dewpoint temperature in the exhaust system is exceeded; and depositing at least some of the liquid on the particulate matter sensor.
By injecting liquid (e.g., urea) into the exhaust system it is possible to saturate a particulate filter, SCR, and/or catalyst with the liquid so that at least a portion of the injected liquid deposits on the particulate matter sensor so that operation of the particulate matter sensor may be verified. Operation of the particulate sensor may be verified if the conductivity of the particulate matter sensor changes (e.g., increases) in the presence of the liquid. Otherwise, if the conductivity of the particulate matter sensor changes by less than a predetermined amount, it may be determined that the particulate matter sensor is degraded. Thus, liquid may be injected into the exhaust system to the extent that the storage capacity of exhaust system components is exceeded such that there may be a higher possibility of depositing liquid on the particulate matter sensor, thereby improving the reliability of the diagnostic method.
The present description may provide several advantages. Specifically, the approach provides a way to diagnose operation of a particulate matter sensor even when the particulate matter sensor is located downstream of a device that holds exhaust constituents. Further, the approach provides an active way for diagnosing operation of a particulate matter sensor. For example, the method provides for outputting a signal in the form of a liquid that can directly stimulate the output of the particulate matter sensor. Further still, in one example, the approach can conserve liquid injected into the exhaust system by first attempting to diagnose the particulate matter sensor with liquid that may already be in the exhaust system. Injection of the liquid external to the exhaust system may commence only after conductivity of the particulate matter sensor fails to change by a predetermined amount in response to any liquid that may be within the exhaust system.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.